The present invention is generally directed to systems and methods for reducing noise in touch screen systems and, more specifically, to a system and method for reducing noise in a touch screen by synchronously triggering a microcontroller to record touch screen location data signals after the expiration of a period of time in which noise occurs.
A touch screen system is a computer input system capable of receiving input signals for a computer through a pressure sensitive plate. When an input stylus, or a pen, or a finger touches the pressure sensitive plate (the xe2x80x9ctouch screenxe2x80x9d) at a point on the surface of the touch screen, the touch screen system senses the location of the xe2x80x9ctouch pointxe2x80x9d within the area of the touch screen. The touch screen system sends information concerning the location of the xe2x80x9ctouch pointxe2x80x9d to a computer.
The computer is programmed to associate specific locations on the touch screen with certain predetermined input signals. For example, touching one area of the touch screen may instruct the computer to perform a certain function and touching another area of the touch screen may instruct the computer to perform another function.
A touch screen system is usually used in conjunction with a video display device. The touch screen is made of transparent material. When the touch screen is placed over a video display device, images from the video display device are visible through the touch screen. The computer displays images on the video display device that identify various instructions or input signals that the computer will receive when the areas of the touch screen associated with those images are touched.
Touch screen systems may be used in conjunction with a wide variety of video display devices. Touch screens may be used on large size video display units such as those used in cash register displays in restaurants or other retail establishments. Touch screens may also be used on small size video display units such as those used in hand-held personal digital assistants. The small size video display devices that are most commonly used with touch screen systems are liquid crystal display devices.
One well known type of touch screen system uses an xe2x80x9canalog resistivexe2x80x9d touch screen. In an analog resistive touch screen, a voltage is applied across a resistor network that underlies the touch screen. The resistance of the resistor network changes when the touch screen is touched. An identifiable voltage change may be detected as each individual xe2x80x9ctouch pointxe2x80x9d is touched. The touch screen system measures the change in resistance of the resistor network in order to determine the location of xe2x80x9ctouch pointxe2x80x9d on the touch screen that is being touched.
The performance of analog resistive touch screens suffers from errors due to noise. Noise on an analog resistive touch screen may be introduced from a variety of sources. One external noise source is the Liquid crystal display itself. As a liquid crystal display operates, it generates a certain amount of noise. This noise is coupled to the touch screen that overlays the liquid crystal display. Another external noise source is the backlight inverter that is used to illuminate the liquid crystal display.
One prior art approach for reducing noise in a large size touch screen involves placing a relatively thick piece of glass between the liquid crystal display and the touch screen. Because the amplitude of the noise signal decreases as the square of the distance from the noise source, increasing the distance of the touch screen from the liquid crystal display reduces the effect of the noise on the touch screen. The glass is used to keep the touch screen a fixed distance from the liquid crystal display. Alternatively, an air gap may be used to accomplish the same result. The relative thickness required for the glass (or the air gap) makes this approach impractical for small size touch screens. It is noted that this approach does nothing to reduce the noise level at the source of the noise.
Another prior art approach for reducing noise involves using a specially treated layer of glass between the touch screen and the liquid crystal display. The layer of glass is coated with a conductive material to provide electromagnetic shielding to reduce noise. The coating, however, reduces the amount of light that is transmitted to the touch screen. This approach also suffers from the disadvantage that it is relatively expensive.
Another prior art approach for reducing noise in touch screens involves adding noise filter capacitors to the sense lines of the touch screen. However, there are several problems with this approach. The noise reduction process is slow (i.e., it does not reduce noise sufficiently quickly), it increases power consumption, and it increases the time required for a signal to stabilize or xe2x80x9csettlexe2x80x9d to a particular value. In addition, the noise filter capacitors themselves also introduce some error.
There is therefore a need in the art for an improved system and method for reducing noise in touch screen systems.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide an improved system and method for reducing noise in touch screen systems.
A touch screen system comprises an analog to digital converter capable of converting to digital signals analog signals from at least one analog resistive film in a touch screen. The analog signal is indicative of a location on the touch screen. The present invention comprises a system and method for reducing noise in a touch screen system comprising a microcontroller coupled to the analog to digital converter. The microcontroller receives a synchronization trigger signal, and in response to receiving the synchronization trigger signal, waits to receive digital signals from the analog to digital converter until the expiration of a period of time in which noise associated with the synchronization trigger signal has occurred.
The present invention also comprises a method for reducing common mode noise on the touch screen by actively sensing and removing the ground return offset from the touch screen data signals. An alternate method for reducing common mode noise comprises removing common mode noise from touch screen data signals by holding the touch screen data signals in a sample and hold circuit.
It is a primary object of the invention to provide a system and method for reducing noise in touch screen systems.
It is also an object of the invention to provide a system and method for reducing common mode noise in touch screen systems.
It is another object of the invention to provide a microcontroller capable of receiving a synchronization trigger signal, and in response to receiving the synchronization trigger signal, capable of waiting to receive touch screen data signals until the expiration of a period of time in which noise occurs.
It is yet another object of the invention to provide a system and method capable of reducing touch screen noise from a noise level of plus or minus ten pixels to a noise level no greater than plus or minus one pixel.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.